This application is based on Patent Application No. 2000-301096 filed Sep. 29, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet printing apparatus and in ink jet printing method, and more particularly to a system of driving an electrothermal transducer to apply thermal energy to ink so as to generate a bubble and, by a pressure of the bubble, eject an ink droplet.
2. Description of the Related Art
Printing apparatus represented by printers have been in wide use in recent years. There has been a growing demand for such printing apparatus to have capabilities of faster printing speed, higher print resolution and lower noise. Among printing apparatus that meet such requirements is an ink jet printing apparatus. The ink jet printing system is a system that ejects ink droplets (printing liquid) from ejection openings of a printing head onto a printing medium and cause the ejected ink droplets to be deposited on the printing medium to perform printing. This system can realize the fast printing and other features described above relatively easily and, because the printing is done without contact between the printing head and the printing medium, fixing of ink to the printing medium is not disturbed thus assuring the printing of a relatively stable image.
Of the ink jet printing systems, a system that uses thermal energy generated by the electrothermal transducer to eject ink is widely used. This system generates thermal energy by applying a driving signal of a predetermined voltage across an electrothermal transducer (hereinafter referred to also as a xe2x80x9cheaterxe2x80x9d).
Heaters and wiring electrodes for applying voltage to the heaters are fabricated on a substrate by using the same technology as used in the semiconductor manufacturing process, and from this substrate, a printing head is made. Heating resistor films each forming the individual heater provided in each ejection opening of the printing head, for example, have variations in manufacturing of the heating resistor, which in turn may cause variations in the resistance values. Hence, even when the same voltages of signals are applied to the heaters of the printing head, the resistance variations result in current variations among heaters. This in turn causes variations in the thermal energy generated and therefore some ejection openings may fail to eject ink properly. Further, even when there are no variations among the heaters in one printing head, there may be variations among different printing heads.
To deal with this problem, a conventional practice adopted in the manufacturing process involves measuring resistance values of a plurality of heaters in the printing head in advance and, based on the resistance measurements, setting pulse widths of driving pulses applied to individual heaters. Furthermore, the pulse widths are determined by taking into account the resistances of wiring electrodes as well as the heater resistances.
Regarding the driving of a multi-nozzle head having a plurality of ejection openings (hereinafter referred to also as xe2x80x9cnozzlesxe2x80x9d), a so-called time-division driving (or block driving) is known. A simplest control method of printing a line along a direction in which the nozzles are arranged is to simultaneously eject ink from all the nozzles of the printing head. When the printing head has a large number of nozzles for fast printing and high print resolution, however, simultaneous driving of all the nozzles of the printing head may cause a significant voltage drop or create a temporary large negative pressure in a common liquid chamber making it difficult to refill ink into individual nozzles as quickly as required. To deal with this problem, the time-division driving system is often employed whereby a plurality of nozzles in the printing head are divided into several blocks and the driving of the printing head is performed for each block on a time-division basis. With this time-division driving system, ink dots form by ink droplets ejected from the one block of nozzles have some positional deviation from ink dots formed by other block of nozzles. This deviation is made as indistinguishable as possible by adjusting the positions of the nozzles in the printing head or by tilting columns of nozzles.
The number of nozzles of the printing head may be set to as large as several hundred or several thousand nozzles and the heater driving frequency may be set to several tens of kHz so as to meet further demands for faster printing and higher resolution. In that case, the number of heaters that need to be driven simultaneously in each block increases and thus an instantaneous maximum current also increases, further increasing the drop in the power supply voltage due to wiring electrodes. Although the number of heaters driven simultaneously changes according to print data, when the number of heaters in each block is large as described above, the relatively large voltage drop prevents individual heaters from being supplied a required voltage for ink ejection, thus resulting in an ink ejection failure such as ink being not ejected or an insufficient amount of ink being ejected.
To solve this problem, a conventional practice is to minimize the wiring resistance and increase a set voltage for the heater driving signal so as to be able to tolerate the maximum voltage drop.
With the above method of increasing the set voltage, however, since there is a limit to the voltage that the heater can withstand, the set voltage cannot simply be increased according to an increase in the number of heaters. Further, when the number of heaters to be driven simultaneously is small depending on the print data, the large set voltage applies an excess energy to the heaters, lowering the thermal efficiency and degrading the durability of the heaters.
To solve this problem, a method is known which counts the number of heaters to be driven simultaneously and controls the pulse width and voltage of the driving signal, as disclosed in Japanese Patent Application Laid Open No. 9-11504. In more detail, this method counts the number of heaters to be driven simultaneously, calculates the voltage drop based on this count, and controls the pulse width and voltage according to the calculated voltage drop. This can prevent the above-described ejection failure or faulty ejection. Because an appropriate pulse width or voltage value calculated on the basis of the number of heaters to be simultaneously driven is set, this method is advantageous in terms of thermal efficiency and the heater durability.
The voltage control in this method, however, is not practical. This is because compensating for the voltage drop requires a high-precision and fast control of voltage and applying this control to the currently known voltage control power supply not only raises cost but is technically difficult. Hence, it is a common method to control only the pulse width to compensate for that part of the bubble generating energy corresponding to the voltage drop caused by simultaneous driving.
As described above, a generally employed practice is to control the pulse width of the heater driving signal in order to solve the ejection failure problem caused by variations in wiring resistance associated with heater driving and by voltage drop due to simultaneous driving of a plurality of heaters.
The pulse width control described above, however, has a problem that the pulse width itself may become too large to match the driving frequency or that the control range of pulse width may become wide causing variations in the amount of ink ejected and the ink ejection velocity.
FIG. 1 is a graph showing a relation between a pulse width of the heater driving signal and an amount of ink ejected. This relation is obtained under the condition that the drive signal is a single rectangular pulse, that the pulse voltage is set constant, and that the pulse energy from which the amount of energy corresponding to the voltage drop is subtracted and which actually contributes to the ink ejection is made constant regardless of the pulse width. That is, the pulse energy has a constant ratio (larger than 1) in magnitude to a bubble generation critical energy whatever the pulse width, the bubble generation critical energy being a limit energy at which a bubble is created.
As shown in FIG. 1, the amount of ink ejected greatly varies in a range where the pulse width is relatively small. None of the conventional pulse width control methods described above uses this largely varying region, and the pulse width control is performed by elongating a basic pulse width and using a service range where the ink ejection amount variation is small. This makes it possible to prevent the ink ejection amount and the ink ejection speed from changing even when the pulse width is changed by the control.
When the printing head is driven at higher speed, the drive cycle or period becomes shorter and the pulse of relatively long width fails to fit in the short drive period, causing trouble to the driving of the printing head. Further, when the pulse width control is to be carried out appropriately according to a large voltage drop, the control width necessarily becomes large, with the result that the region shown in FIG. 1 where the ink ejection amount varies greatly may be included in the control range. Hence, the pulse width control using this region will cause variations in the ink ejection amount and speed, significantly degrading the quality of a printed image.
An object of the present invention is to provide an ink jet printing apparatus and an ink jet printing method which, when controlling a pulse width according to a change in heater resistance and wiring resistance in a printing head and to a change in a voltage drop caused by simultaneous driving of a plurality of heaters, can properly determine a control range of the pulse width to ensure a stable ejection of ink.
In a first aspect of the present invention, there is provided an ink jet printing apparatus using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the apparatus comprising:
detecting means for detecting a quantity indicating an amount of voltage drop of the driving signals that occurs when the driving signals are supplied to the plurality of heaters;
obtaining means for obtaining a conduction period for the heaters in the case that the driving signals are a form of single pulse, in accordance with the quantity detected by the detecting means; and
changing means for changing the driving signals into divided signals in accordance with the conduction period obtained by the obtaining means.
In a second aspect of the present invention, there is provided an ink jet printing apparatus using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the apparatus comprising:
detecting means for detecting a quantity indicating an amount of voltage drop of the driving signals that occurs when the driving signals are supplied to the plurality of heaters; and
control means for controlling the driving signals in accordance with the quantity detected by the detecting means so that the smaller the predetermined quantity is, the longer a pulse width of a previous pulse in divided pulses as the driving signal is.
In a third aspect of the present invention, there is provided an ink jet printing apparatus using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the apparatus comprising:
detecting means for detecting a quantity indicating an amount of voltage drop of the driving signals that occurs when the driving signals are supplied to the plurality of heaters; and
control means for controlling the driving signals in accordance with the quantity detected by the detecting means so that the smaller the quantity is, the greater a rate of a pulse width of a previous pulse in divided pulses as the driving signal is.
In a fourth aspect of the present invention, there is provided an ink jet printing apparatus using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the apparatus comprising:
detecting means for, when the driving signals are supplied to the plurality of heaters, detecting a number of heaters to which the driving signals are supplied simultaneously;
obtaining means for obtaining a conduction period for the heaters in the case that the driving signals are a form of single pulse, by referring to a table with the number of heaters detected by the detecting means; and
determining means for determining a waveform of a pulse as the driving signal by referring a division table with the conduction period obtained by the obtaining means.
In a fifth aspect of the present invention, there is provided an ink jet printing method of using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the method comprising the steps of:
detecting a quantity indicating an amount of voltage drop of the driving signals that occurs when the driving signals are supplied to the plurality of heaters;
obtaining a conduction period for the heaters in the case that the driving signals are a form of single pulse, in accordance with the quantity detected by the detecting step; and
changing the driving signals into divided signals in accordance with the conduction period obtained by the obtaining step.
In a sixth aspect of the present invention, there is provided an ink jet printing method of using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the method comprising the steps of:
detecting a quantity indicating an amount of voltage drop of the driving signals that occurs when the driving signals are supplied to the plurality of heaters; and
controlling the driving signals in accordance with the predetermined quantity detected by the detecting step so that the smaller the predetermined quantity is, the longer a pulse width of a previous pulse in divided pulses as the driving signal is.
In a seventh aspect of the present invention, there is provided an ink jet printing method of using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the method comprising the steps of:
detecting a quantity indicating an amount of voltage drop of the driving signals that occurs when the driving signals are supplied to the plurality of heaters; and
controlling the driving signals in accordance with the quantity detected by the detecting step so that the smaller the predetermined quantity is, the greater a rate of a pulse width of a previous pulse in divided pulses as the driving signal is.
In a eighth aspect of the present invention, there is provided an ink jet printing method of using a printing head, which applies driving signal to a plurality of heaters for generating thermal energy so that ink is ejected by utilizing the thermal energy, to eject the ink to a printing medium for performing printing, the method comprising the steps of:
when the driving signals are supplied to the plurality of heaters, detecting a number of heaters to which the driving signals are supplied simultaneously;
obtaining a conduction period for the heaters in the case that the driving signals are a form of single pulse, by referring to a table with the number of heaters detected by the detecting step; and
determining a waveform of a pulse as the driving signal by referring a division table with the conduction period obtained by the obtaining step.
With the construction above, the driving signal is changed from a single pulse to a double pulse or a desired pulse waveform is determined, according to a predetermined amount indicating an amount of voltage drop of the driving signal that occurs when the drive signal is supplied to a plurality of heaters. This enables the heaters to be driven with a double pulse or a changed waveform pulse of the driving signal if the heaters are driven with a single pulse, a pulse width, or conduction time, of which is in a range that varies the ink ejection amount. It is therefore possible to eject ink with the drive signal that does not cause variations in the ink ejection amount.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.